I really don't buy this, its mostly a cultural fad. You can design your system as multiple services interacting via a strict API even in a monolith. Failure isolation is a pattern of how to handle errors, whether they come across as a 40x HTTP status code, a JSON field called "error" or a method call throwing an exception. And a single system enables you to work with simple method calls calling across services instead of all the complication SOA can bring with signaling errors/serialization over a network layer. Not to mention all the operational overhead that comes with managing more hosts and more services.
You make the assumption that failures will be covered by defensive programming techniques. Unexpected failures will take down a monolith. One bad db query can deadlock all consumers. The greater the separation between components, the more easily you can isolate all failures, not just expected ones.
